Synthesis and electrochromic properties of conducting polymers based on highly planar 2,7-disubstituted xanthene derivatives

Article abstract of DOI:10.1002/cplu.201402349

On the basis of preliminary DFT calculations, p-type semiconducting polymers based on 2,7-substituted xanthene building blocks that show a high degree of planarity were designed. The synthesis, electrochemical characterization, and theoretical modeling of 2,7-bis(thiophen-2-yl)-9,9-dimethylxanthene (1) and 2,7-bis(3-hexylthiophen-2-yl)-9,9-dimethylxanthene (2) is described. The synthetic procedure is based on the incorporation of thiophene rings by means of Pd-catalyzed cross-coupling reactions, which lead to monomers 1 and 2. Copolymers P1 and P2 obtained by means of anodic polymerization have been characterized by spectroscopic and electrochemical methods. Electrochromism was observed for both polymers. The experimental data supported by density functional theory modeling explain the influence of alkyl chain substitution on the properties of the investigated copolymers.

Full text of DOI:10.1002/cplu.201402349

DOI: 10.1002/cplu.201402349
Full Papers
Synthesis and Electrochromic Properties of Conducting
Polymers Based on Highly Planar 2,7-Disubstituted
Xanthene Derivatives
[
a]
[b]
[b]
[a]
Kamila Olech, Ramona Gutkowski, Volodymyr Kuznetsov, Szczepan Roszak,
[a]
[b]
Jadwiga Sołoducho,* and Wolfgang Schuhmann*
On the basis of preliminary DFT calculations, p-type semicon-
ducting polymers based on 2,7-substituted xanthene building
blocks that show a high degree of planarity were designed.
The synthesis, electrochemical characterization, and theoretical
modeling of 2,7-bis(thiophen-2-yl)-9,9-dimethylxanthene (1)
and 2,7-bis(3-hexylthiophen-2-yl)-9,9-dimethylxanthene (2) is
described. The synthetic procedure is based on the incorpora-
tion of thiophene rings by means of Pd-catalyzed cross-cou-
pling reactions, which lead to monomers 1 and 2. Copolymers
P1 and P2 obtained by means of anodic polymerization have
been characterized by spectroscopic and electrochemical
methods. Electrochromism was observed for both polymers.
The experimental data supported by density functional theory
modeling explain the influence of alkyl chain substitution on
the properties of the investigated copolymers.
Introduction
Semiconducting organic materials possess broad application
potentials in layered, microscopic electronics: for example, as
those properties. Thus, the development of synthesis methods
as well as the selection of suitable building blocks are priori-
tized approaches in the design of low-bandgap polymer com-
posites.
[1]
light-emitting diodes, photovoltaic devices, and sensors. In
this context, electronically conducting polymers that demon-
strate high conductivity upon doping are of particular interest.
Chemical or electrochemical doping leads to the formation of
solitons, polarons, or bipolarons, which are delocalized over
Despite the fact that many heteroaromatic compounds have
been explored, comparatively little attention has been devoted
to xanthene derivatives; mainly materials based on rhoda-
[
2]
[5]
[6]
[7]
the polymer molecules and act as charge carriers. The optoe-
lectronic properties, processability, and functionality of organic
semiconducting materials are readily tunable by changing
mine, spiroxanthene, and dibenzoxanthene have been in-
vestigated. The first report on xanthene-based polymers was
[8]
published in 2010 by Morisaki et al., who described 4,5-sub-
stituted xanthene oligomers as highly thermally stable, layered,
and hole-transporting materials. More recently, 9,9-dimethyl-
xanthene was proposed as a bridge for perylene multichromo-
[
3]
their composition and molecular weight.
A convenient approach to obtain organic materials with suit-
able bandgaps is to design and synthesize donor–acceptor
[9]
(DA) type macromolecules. The alternate incorporation of elec-
phore covalent dimers. To the best of our knowledge, 2,7-
heterocycle-substituted xanthene derivatives and their electro-
polymerization have not yet been reported.
tron-rich and electron-deficient units into the molecular back-
bone decreases the HOMO–LUMO energy gap owing to the
[
4]
expansion of the delocalized p-electron orbital. The electron-
ic properties are strongly influenced by the chemical composi-
tion of related copolymers. The steric factors of polymer matrix
formation and matrix defects also contribute considerably to
In this study, we present the synthesis, electrochemically in-
duced polymerization, and characterization of 2,7-substituted
xanthene derivatives, which show a high degree of planarity
that can significantly affect the electrical and optical properties
of related oligomers or polymers. Specifically, we investigated
2,7-bis(thiophen-2-yl)-9,9-dimethylxanthene (1) and its alkylat-
[
a] K. Olech, Prof. Dr. S. Roszak, Prof. Dr. J. Sołoducho
Faculty of Chemistry
Wrocław University of Technology
Wybrze z˙ e Wyspia n´ skiego 27
ed analogue (2) as precursors for new electrochromic semicon-
ducting materials. The initial evaluation of the physicochemical
properties of the corresponding polymers films P1 and P2
were performed by cyclic voltammetry, electrochemical impe-
dance spectroscopy, and spectroelectrochemical measure-
ments. Experimental results were supported by theoretical
modeling based on density functional theory (DFT). The role of
the alkyl chain with respect to the optoelectronic properties of
electronically conducting xanthene polymers is discussed on
the basis of both the calculated and atomic force microscopy
(AFM) structure visualization.
5
0-370 Wrocław, Poland
E-mail: jadwiga.soloducho@pwr.edu.pl
[
b] R. Gutkowski, Dr. V. Kuznetsov, Prof. Dr. W. Schuhmann
Analytical Chemistry–Center for Electrochemical Sciences (CES)
Ruhr-Universität Bochum, Universitätsstrasse 150
4
4780 Bochum (Germany)
E-mail: wolfgang.schuhmann@rub.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201402349.
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Compounds 1 and 2 were obtained with 42 and 31% yields,
respectively. They exhibit good solubility in common organic
solvents such as chloroform and acetonitrile. The possibility of
their synthesis by a convenient solution-processable technique
or thermal evaporation at room temperature can be consid-
ered an advantage over inorganic-based semiconductor devi-
Results and Discussion
Synthesis of 1 and 2
The synthesis of monomers 1 and 2 is presented in Scheme 1.
A variety of condensation reactions was investigated to sym-
metrically substitute the xanthene moiety with thiophene
rings. Kumada-type coupling was found to be the least effi-
cient as it uses air- and moisture-sensitive Grignard reagents,
which were unstable over the reaction time. Thus no reaction
progress was observed after one day according to thin-layer
chromatography (TLC), whereas in the reaction mixture a high
concentration of unreacted educts was detected. The long-
time synthesis with organotin species according to the Stille
protocol gave about 45% of crude products with impurities
that were difficult to separate chromatographically. The latter
were characterized by a retention factor close to that of the
xanthene derivative (1).
[1b]
ces.
Electrochemical properties
Cyclic voltammetry (CV) of solutions of monomers 1 and 2 in
acetonitrile (both 0.50 mm) was carried out to create thin films
of the corresponding copolymers P1 and P2. As can be seen
from the voltammograms, both synthesized monomers are
electroactive (Figure 1). The oxidation potential is lower for
thiophene then the 3-hexylthiophene ring and is 1.29 and
1.34 V for monomers 1 and 2, respectively. This redox process
is associated with the monoelectronic oxidation of the mono-
[12]
The Suzuki–Miyaura reaction in mixed aqueous/ethoxyetha-
nol solvent (1:9), reported previously for the coupling of thio-
mers to the corresponding radical cations. The appearance
of new oxidation peaks at 0.75 (in the case of P1) and 1.00 V
(in the case of P2) after repetitive anodic scans clearly indicates
the formation of electroactive oligomer films on the electrode
surfaces. The slow charge transfer within the film and/or at the
interfaces is illustrated by the broadening of the redox
[
10]
phene with different arenes, was found to be the most suc-
cessful synthesis pathway when considering both efficiency
and reaction time. An additional advantage is the nontoxicity
[11]
of byproducts.
[13]
waves. The lower intensity of
the cathodic peak at 1.18 V sug-
gests the consumption of radical
cations during the polymeri-
zation reaction. Hence the in-
crease in polymer thickness on
the Pt electrode after each cycle
is seen as an increase of the
peak current. The redox conver-
sion at about 0.1 V for P1 in
both the anodic and cathodic
half-scans (Figure 1) indicates
the formation of a quasi-reversi-
Scheme 1. Synthesis of 2,7-disubstituted xanthene derivatives 1 and 2. a) Me
CH COOH; c) [Ni(dppp)Cl ] (dppp=1,3-bis(diphenylphosphino)propane), Et O, 72 h; d) 0.4 mol% [Pd(PPh
CO , EtO(CH OH/H O (9:1), 24–48 h; and e) [PdCl (PPh ], tetrahydrofuran (THF), 48 h.
3 3 2 3 2
Al, PhCH ; b) Br , (CH CO) O,
3
2
2
3 4
) ],
K
2
3
2
)
2
2
2
3 2
)
À1
Figure 1. Cyclic voltammograms of the electrochemically induced polymerization of 1 (left) and 2 (right) in 0.1m Bu
Monomer concentration was 0.50 mm. The potential was referenced against the Ag/AgCl/3m KCl electrode.
4
NPF
6
/MeCN with a scan rate of 50 mVs
.
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ble redox system during polymer deposition. The formation of
P1 and P2 was confirmed by an optically visible green layer on
the Pt surface.
material. Since the charge percolation through the polymer
matrix generally dictates operational characteristics of the de-
[14]
posited microstructure, it can explain the lower R of P2.
ct
The deposited films were additionally characterized by
means of electrochemical impedance spectroscopy (EIS) in the
The stability of the obtained films was investigated by a se-
quential doping/dedoping process in monomer-free electrolyte
3À/4À
À1
presence of [Fe(CN)6]
as a redox couple (c=5 mm). Impe-
using cyclic voltammetry at a scan rate of 300 mVs in the po-
dance spectra were acquired in a frequency range from 60 kHz
to 0.01 Hz with an alternating current (ac) perturbation ampli-
tude of 10 mVrms at a constant direct current (dc) potential of
tential range between À0.40 and +1.35 V and 0.0 and +1.40 V
for P1 and P2, respectively. The height of the oxidation peaks
decreases by approximately 50% after 25 voltammetric cycles
for P1 and after 55 cycles for P2. The higher stability of P2
might be a consequence of the lower solubility owing to the
likely higher molecular weight of the formed polymer chains.
The bandgap as well as oxidation and reduction properties for
the obtained polymers are given in Table 1.
+
210 mV versus the Ag/AgCl/3m KCl reference electrode. The
film grown on the Pt electrode hinders the redox conversion
3
À/4À
of [Fe(CN)6]
, which can be seen by a drastic increase in the
charge-transfer resistance R . The R of P2 is 9.28 MW as deter-
ct
ct
mined from the diameter of the semicircle of the Nyquist (Z’’
versus Z’) plot, in which Z’ and
Z’’ are real and imaginary parts
of the impedance, respectively
Table 1. Calculated and experimental ionization energies (eHOMO, IPver, IPad), calculated and experimental elec-
tron affinity (eLUMO, EAexp, cyclic voltammetry), calculated singlet and triplet electronic excited states (DEexcited),
and corresponding experimental (UV/Vis and cyclic voltammetry) energy gap (DEHOMO/LUMO) between valence
and conduction bands for P1 and P2. All energies are in eV.
(Figure 2).
Compound P1 shows a much
higher Rct than P2. This is sur-
prising, because the thickness of
the P1 film (60–90 nm) is consid-
erably smaller than that of the
P2 film (150–250 nm) as deter-
mined by AFM (see below). This
effect could be attributed to the
fact that the multilayer film of
P2 is most likely more porous
owing to the presence of n-
hexyl chains in the polymer
backbone, which increase the
distance between molecules.
Therefore, charges from the elec-
trolyte are better dispersed
within the redox-active polymer
n
1
2
3
4
5
Monomer
Polymer
Theoretical
Experimental
[
a]
P1
P2
e
HOMO
À5.42
6.66
6.51
À1.25
4.17
3.73
2.69
À5.53
6.70
6.49
À0.93
4.59
À5.07
6.02
5.89
À1.83
3.24
2.89
1.89
À5.15
6.08
5.89
À1.47
3.67
À5.03
5.77
5.68
À1.89
3.13
–
À5.02
5.63
5.56
À1.92
3.10
–
À5.01
5.53
5.48
À1.94
3.07
–
–
À5.47
À5.47
À5.47
À2.77
2.70
[
[
a]
a]
[a]
[a]
[a]
IPver
IPad
e
5.95
5.95
–
LUMO
[a]
DEHOMO/LUMO
DEexcited singlet
DEexcited triplet
–
[
b]
b]
[b]
[b]
3.64
3.64
–
6.00
6.00
–
–
3.83
3.83
2.67
2.67
[
–
–
–
[a]
e
HOMO
À5.11
5.82
5.73
À1.52
3.59
–
À5.08
5.67
5.60
À1.54
3.54
–
À5.08
5.58
5.64
À1.56
3.52
–
À5.59
À5.59
À5.59
À2.78
2.81
[a]
[a]
[a]
IPver
[
a]
IPad
e
[
[
a]
a]
LUMO
DEHOMO/LUMO
DEexcited singlet
DEexcited triplet
[b]
[b]
[b]
4.06
3.04
3.27
3.03
2.78
2.78
[b]
–
–
–
[a] Experimental electrochemical. [b] Experimental UV/Vis spectroscopy.
The energy gaps of P1 and P2 deposited on a platinum elec-
trode were derived from cyclic voltammetry measurements re-
corded in acetonitrile that contained 0.1m Bu NPF at a scan
4
6
À1
rate of 100 mVs (Figure 3). The distance between the irrever-
sible oxidation and reduction potentials, which are correlated
[15]
with the HOMO and LUMO energies, respectively,
.70 eV for P1 and 2.81 eV for P2.
are
2
Spectroelectrochemical properties of polymers
The chemical changes that occur in the polymer films during
the oxidative doping process were investigated by means of
UV/Vis spectroelectrochemistry. In the negative potential range
an absorption band with a maximum at 318 nm was observed
for P1 (Figure 4). For P2 two absorption bands were recorded,
which can be related to oligomers with significantly different
chain lengths. The first band is close to the monomer absorp-
tion band and corresponds to short oligomers, whereas the
second less-intense band at 365 nm is caused by longer poly-
mer chains (Figure 4).
Figure 2. Nyquist plots of a bare Pt electrode (inset; *) and after electro-
chemically induced polymerization of 1 (D) and 2 (^). EIS was recorded in
3À/4À
the presence of 5 mm [Fe(CN)
6
]
at +210 mV versus Ag/AgCl/3m KCl;
perturbation amplitude: 10 mVrms and frequency range: 60 kHz to 0.01 Hz.
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were investigated by atomic force microscopy. Representative
AFM images of two anodically polymerized films P1 and P2
are shown in Figure S1 of the Supporting Information.
Both materials were obtained from solutions of identical mo-
nomer concentration under similar electrochemical conditions.
They mask, to some extent, the underlying topography of the
polished Pt electrodes. However, their z-color scale bars that
represent the overall topography variations differ considerably.
For P1 it corresponds to 100 nm, whereas for P2 to 800 nm.
The root-mean-square (rms) roughness of P1 (approximately
2
1
6 nm over 100 mm ) is about 18 times lower than that of P2
2
(
approximately 286 nm over 100 mm ). The difference in the
apparent topography of P1 and P2 might originate from
a number of factors, including the variation in interchain inter-
action owing to different steric constrains, the variation of the
polymerization degree, and hence of the polymer structure.
These factors contribute not only to the surface roughness but
also to the thickness of the polymer film. As determined by
AFM scratching and subsequent imaging, the thickness of P1
is about 60–90 nm, whereas it is 150–250 nm for P2. The differ-
ence in thickness might indicate that the formation of the
polymer occurs with a simultaneous blocking of the electrode
surface in the case of P1 and with a minor blocking in the case
of P2. This qualitative behavior is in accordance with the re-
sults obtained by impedance spectroscopy.
Figure 3. The electrochemical doping process of polymer P1 and P2 in 0.1m
À1
Bu
4
NPF
6
/MeCN at a scan rate of 100 mVs
.
These absorption bands characterize the neutral, dedoped
state, which corresponds to the p–p* electron transition from
[
16]
the valance band to the conduction band. Oxidation of the
polymers led to the appearance of a new broad absorption
band while simultaneously decreasing the intensity of the
bands at lower wavelengths. The bands in the range of 500–
8
00 nm can be attributed to the polaron generated during
[
17]
electrochemical doping.
Well-defined isosbestic points at
4
43 nm for P1 and at 434 nm for P2 reflect the transition of an
Computational study: Structural considerations
insulating state into a conducting one. These transformations
during oxidation were additionally evidenced by the polymer
color change. A change in color from yellow (reduced form) to
green and blue (oxidized forms) was observed for both P1 and
P2.
The comparison of structural data for dimers of 1 and its ana-
logue substituted with thiophene rings in the 4,5-positions
clearly shows that the value of the dihedral angle between the
thiophene and xanthene planes is crucial for the high conduc-
tivity achieved owing to close orbital overlapping. Therefore,
to minimize steric effects, the heteroaromatic rings were di-
rectly attached to the xanthene moiety in the 2- and 7-posi-
tions.
Thin-film morphology
3
Influences of the polymer composition on the thin-film mor-
phology, which can determine the charge-transport properties,
Despite the presence of an sp -carbon atom (of the C(CH )
3
2
group) and oxygen with two single atoms in the central xan-
Figure 4. UV/Vis spectra recorded during electrochemical oxidation of polymer films P1 (left) and P2 (right).
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thene ring, the molecular skeleton is planar (Scheme 1). The C-
O-C angle of about 1208 indicates the involvement of oxygen
orbitals in the p-electron conjugation of the molecule (possible
The evolution of the molecular orbital space
In monomers the eight highest-occupied molecular orbitals
possess p character. The orbitals are delocalized and cover the
whole skeleton and include the perpendicular p orbital of the
2
sp hybridization). The C-C-C-S dihedral angle between the
xanthene and thiophene planes in P1 amounts to 28.58, thus
indicating the lack of conjugation between these rings. The di-
merization of 1 leads to a new CÀC bond formed between two
thiophene rings. This link only slightly influences the mono-
meric building blocks. The new structural fragment is charac-
terized by a CÀC bond length of 1.447 Š and an S-C-C-S dihe-
oxygen, CÀCH bonds of the xanthene center, as well as single
3
CÀC xanthene–thiophene bonds (Figure 5). Six lowest orbitals
of the virtual space also possess p character. The orbitals are
strongly delocalized, and the p character is not significantly
perturbed despite the imperfect planarity of the molecular
skeleton. Dimerization results in the formation of a new, for-
mally single CÀC bond between the thiophene rings
(Scheme 1). This bond, however, is covered by the p orbital lo-
calized on the new structural element (Figure 5). This orbital
constitutes the highest occupied molecular orbital and belongs
to the family of 16 orbitals of p character. The virtual space
consists now of 12 p orbitals with the LUMO orbital being lo-
calized in the area of the newly formed bond. Further exten-
sion of the oligomer does not result in qualitative changes.
The HOMO/LUMO p space is multiplied, which leads to, for ex-
ample, 40 (occupied) and 30 (virtual) orbitals in the case of the
pentamer and constitutes the base for future polymer bands.
Four HOMO and four LUMO orbitals correspond to orbitals of
thiophene–thiophene links, and might be considered a building
block for valence and conduction bands of P1. As expected,
dral angle of 169.58. Further oligomerization (P1 , n=3–5)
n
does not change the C-C-C-S angle and the thiophene–thio-
phene bonds also remain unchanged. The only variation ob-
served regards the S-C-C-S dihedral angles between thiophene
rings, which increase from 169.58 in the dimer to 1778 in the
pentamer (P1 ). These observations indicate the almost perfect
5
structural additivity of building blocks during the formation of
long P1 oligomers. Ionization leads to a completely planar
structure of 1 and P1 . Complexes P1 are also planar with the
2
3
exception of external thiophene rings being slightly twisted by
about 158.
To overcome the low solubility previously noted for many
high-molecular-weight semiconducting compounds, alkyl
chains were integrated. The modification of P1 by ÀC H sub-
6
13
stitution leads to a significant C-C-C-S angle variation. Its in-
crease to up to 528 indicates the
loss of p-electron conjugation.
The thiophene–thiophene dihe-
dral angle between subsequent
mers decreases from 1668 in the
dimer and approaches planarity
for P2 . Other structural parame-
5
ters remain almost unchanged,
which suggests the structural
additivity of the
2 building
blocks. The effect of the electron
loss is significant for short oligo-
mers and results in a more
planar structure for the mono-
mer and dimer (C-C-C-S of 278
and S-C-C-S of 1808 for the
dimer). Owing to charge delocal-
ization the effect starts to disap-
pear for longer oligomers. The
main difference between P1 and
P2 oligomers regards xanthene–
thiophene dihedral angles. The
decrease in p conjugation owing
to the structural perturbation
imposed by alkyl chains might
have a significant impact on the
electronic density organization
of the studied moieties. The
structural changes influence the
theoretical energy-gap value,
which is higher for P2 by
Figure 5. Molecular orbitals obtained from B3LYP/cc-pVDZ studies for the neutral form of 1 and P1
and P2
2
as well as 2
0.43 eV.
2
.
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the molecular orbital that corresponds to the s CÀC bond is lo-
cated deep in the orbital space.
À0.520 electrons on the O atom. The atomic charge on
a carbon atom that takes part in the new CÀC bond formation
is totally balanced by its neighbor S. This finding indicates the
importance of the heteroatom for the polymerization reaction.
The electron removal leads to a highly delocalized positive
charge distribution with the difference between atomic charg-
es in the cation and parent molecule systematically decreasing
as the oligomer grows. The change for the most effected
atoms in monomer 1 are 0.054 (S), 0.067 (a-C), and 0.073 elec-
trons (O) relative to 0.038 (S), 0.023 (a-C), and 0.026 (O) elec-
Although structural changes occur owing to the attached
alkyl chains in the P2 derivative, the qualitative picture of mo-
lecular orbitals (Figure 5) and the variation of energies
(Figure 6) are preserved. The extended HOMO/LUMO orbital
trons in its dimer (P1 ). The trend continues when the oligo-
2
mer grows.
The electron spin density, which represents the a- and b-
electron density difference, is also delocalized (Figure S2 in the
Supporting Information). However, the highest concentration
of unpaired electrons is found on carbon in the a position of
the external ring, which constitutes a site for polymerization.
Since the charge on the carbon in the a position of the dimer
of 2 is slightly lower than the charge localized on its bis-thienyl
part, it can meaningfully impair reactivity. The unpaired elec-
tron distribution suggests that non-regular linkages might
occur. Taking into account the fact that steric effects can influ-
ence the polymerization reaction owing to crowding at or near
the reactive state, carbon atoms of the bis-thienyl moiety in
P2 in particular are rather inaccessible for the radical cation
2
attack. Moreover, it was previously reported for polythiophene
Figure 6. Plots of density of states for P1 and P2 oligomers. The evolution of
orbital energy levels is demonstrated below the plots in the order from
n=1 to 5 of the oligomer growth. Energy in eV, intensity in arbitrary units.
[18]
that thienyl rather does not create net structures. The stabili-
zation of the radical cation is achieved by the delocalization of
the positive charge at longer oligomers, thereby leading to an
increased sensitivity towards oxidation processes, which affects
further polymerization reactions. The electron density in P2 is
2
space possesses p character and increases by eight occupied
and six virtual MOs with each extension of the oligomer by an
additional “building” unit. The observation of the space varia-
tion for increasing oligomers indicates that the valence and
conduction bands are formed from molecular orbitals related
to structural fragments between thiophene–xanthene–thio-
phene blocks.
better distributed over the molecule and it will therefore react
easily to create longer oligomers. Ionization potentials for both
monomers and oligomers (both vertical and adiabatic) are sim-
ilar with a precision of 0.1 eV; they are 6.7–5.6 eV for vertical
and 6.5–5.5 eV for adiabatic.
According to the Koopmans theorem, the HOMO and LUMO
orbital energies represent approximately ionization (HOMO)
and electron affinity (LUMO) energies. The corresponding ex-
perimental values are derived from cyclic voltammetry experi-
ments (see Table 1). The directly calculated DFT values for verti-
cal and adiabatic ionization energies are also reported. Calcula-
tions as well as experiments (oxidation energies) indicate a sig-
nificant change in the ionization energies of the monomer and
polymerized materials. The theoretical explanation of this phe-
nomenon is seen in the properties that arose as new CÀC
bonds formed owing to the polymerization reaction. The most
significant change of properties takes place between monomer
and dimer, whereas further oligomerization is subject to a slow
evolution of properties. The effect is very visible in density-of-
states plots (Figure 6) in which the sequence of HOMO and
LUMO orbital energies that correspond to the bandgap starts
from the dimer molecule. The LUMO orbital energies follow
a similar trend with the monomer/dimer energy jump followed
by a slow evolution of values. Both experimental and theoreti-
cal results indicate the minimal influence of the alkyl chain
that modifies P1.
Despite the fact that the polymerization reaction mechanism
is still under debate, it is agreed that it depends on the elec-
tron-density distribution. As expected for strongly conjugated
molecular systems, the atomic charge as well as electronic spin
density are highly delocalized, and the oxidation effect mea-
sured by the ionization energy quickly decreases with the ex-
tension of the oligomer size. In neutral molecules, charge
transfer between xanthene and thiophene units almost does
not exist. After ionization every heterocyclic ring shows com-
parable electron deficiency. The polymerization does not
change this picture except for the external rings, which do not
lose their electrons in the oxidation process. Those observa-
tions confirm an accepted mechanism of the electrochemical
polymerization in which the polymer grows by the successive
addition of monomers.
Close inspection of natural bond orbital (NBO) atomic charg-
es confirms the additivity of mers in an oligomer. The charge
distribution within the mers is almost the same, with an
atomic charge on the S atom of 0.413 electrons and of
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Theoretical absorption spectra
single bonds. The influence of the alkyl chain on chemical
properties of oligomers is small.
The theoretical absorption spectra were calculated by applying
time-dependent density functional theory (TDDFT) (Figure S3
in the Supporting Information). Ten singlet electronic excited
states were considered. In the case of 1 the lowest singlet ex-
cited state of 332 nm is dominated (c=0.701) by the HOMO–
LUMO excitation. Dimerization leads to the formation of new
HOMO and LUMO orbitals, and the transition between these
orbitals constitutes the main component of the lowest singlet
excitation in the dimer. The corresponding wave function is
dominated by the HOMO–LUMO transition (c=0.699) with
a significant lowering of the excitation energy of 429 nm. The
transition that corresponds to the lowest excitation, which
does not involve the thiophene–thiophene link and that corre-
sponds to the HOMO–LUMO orbitals of the monomer are
much higher in energy (Figure S3), however, constitutes the
second peak of significant intensity. Comparison of the absorp-
tion spectra for P1 and P2 indicates close qualitative similari-
Experimental Section
Materials
2
-Thiopheneboronic acid (95%), 3-hexylthiophene-2-boronic acid
pinacol ester (95%), tetrakis(triphenylphosphine)palladium(0)
99%), 2-ethoxyethanol (99%), tetrabutylammonium hexafluoro-
phosphate (98%), and potassium hexacyanoferrate(II) trihydrate
(
(
(
98.5%) were purchased from Sigma–Aldrich. Potassium carbonate
99%) was obtained from Chempur. Potassium hexacyanoferrate(III)
(99%) was obtained from Honeywell Riedel-de Han. Acetonitrile
(HPLC grade), anhydrous sodium sulfate, and potassium chloride
(99%) were purchased from J. T. Baker. Anhydrous magnesium sul-
fate (98.5%) was received from POCh. All reagents were used with-
out further purification. Preparative column chromatography was
performed on glass columns with Fluka silica gel for flash chroma-
tography (mesh 220–440). Aqueous solutions utilized for electro-
2
2
chemical measurements were prepared using Milli-Q water
À1
ties. The differences are quantitative, which leads to energeti-
cally lower excitation energies, mainly at 305 nm for the mono-
mer and at 379 nm for the dimer. The second peak of signifi-
cant intensity corresponds to the HOMO–LUMO transition of
the monomer.
(
>18 MWcm , Siemens Ultra Pure Water Systems).
Theoretical methods and computational details
The structures of studied molecules and cations were investigated
at the DFT level of theory using a standard cc-pVDZ basis set.
The calculated singlet excitation energies, which are the
measure of the polymer bandgap, agree reasonably well with
the measured UV/Vis spectra. More importantly, calculations
provide the qualitative picture of the evolution (Table 1). The
measured excitation energy in the monomer and its evolution
toward that of the polymer correlates well with the decreasing
excitation energy between monomer and dimer. The electro-
chemical data that represent the valence-conduction bandgaps
agree closely with spectroscopic findings as well as with theo-
retical results. The conductivity of the polymer is controlled by
the chemical bonds formed between monomers. The differen-
ces between the energy-gap properties of P1 and P2 are
rather subtle and the comparison should be treated with cau-
tion.
[19]
[20]
The B3LYP functional was adopted for these calculations. The ge-
ometry of oligomers was fully optimized and the minimum-energy
structures, in the respective cases, were confirmed through fre-
quency calculations. No symmetry was assumed at the optimiza-
tion stage of calculations. The singlet and triplet excited-state cal-
culations were performed by applying time-dependent density
[21]
functional theory (TDDFT). Twenty excited electronic states were
considered for each molecule. The electron distribution was exam-
ined by using NBO and Mulliken electron population analysis
[
22]
schemes.
The calculations were carried out using Gaussian 09
The molecular graphics were generated using Gauss-
[
23]
code.
View 05 and GaussSum software.
[24]
Electrochemical and spectroscopic measurements
All electrochemical measurements were recorded with an mAutolab
III potentiostat with a frequency response analyzer (FRA) (Metrohm
Autolab). Data acquisition and processing were performed using
NOVA1.10 software and OriginPro8.1 (OriginLab). The electrochemi-
Conclusion
2,7-Dithiophene-substituted xanthene derivatives were synthe-
cal cell was comprised of a polycrystalline Pt disc electrode
sized and electrochemically polymerized, thereby leading to
the formation of novel p-type organic semiconducting materi-
als. The introduction of a 3-hexyl chain was shown to effective-
ly enhance performance owing to an increase in the porosity
of the thin film. Theoretical predictions of the oligomer evolu-
tion agree reasonably well with measured data. It was found
that the main difference between the single-molecule building
block and polymer results from the new CÀC bond formed be-
tween thiophene rings. This finding indicates the potential of
the heteroatom (sulfur) to control the conducting properties of
oligomers. The oligomerization has little influence on the struc-
tural properties of building blocks. The HOMO/LUMO orbital
space indicates significant p conjugation despite the far from
perfect planarity and a number of obstacles in the form of
2
(
2.30 mm working area) as a working electrode, an Ag/AgCl/3m
KCl reference electrode, and a platinum coil as auxiliary electrode.
The platinum working electrode was polished consecutively with
diamond slurries (particle size 3, 1, and 0.5 mm) on a polishing
cloth and afterwards was washed for 15 min in water under sonica-
tion. Further cleaning was performed electrochemically by cyclic
À1
voltammetry in 0.5m H
SO
2
at a scan rate of 50 mVs in the po-
4
tential range between 0.2 and 1.1 V. Polymerization was achieved
by cyclic voltammetry (10 cycles) of 0.50 mm monomer solution in
À1
acetonitrile at a scan rate of 50 mVs in the potential range be-
tween À0.40 to +1.35 V and 0.0 to +1.40 V for 1 and 2, respec-
tively. As supporting electrolyte, 0.1m tetrabutylammonium hexa-
fluorophosphate (Bu NPF ) was used. Prior to experiments the elec-
4
6
trolyte solution was purged with argon and an argon atmosphere
was maintained over the solution during the experiment. Electro-
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Full Papers
chemical impedance spectra (EIS) were acquired in an equimolar
chloroform. The crude product was purified by means of column
chromatography (hexane/chloroform) and gave a light green oil of
3À/4À
solution (5.0 mm) of [Fe(CN)6]
at room temperature.
2
(1.30 g, 31%).
Spectroelectrochemical measurements were performed with a Cary
1
3
6
0 UV/Vis spectrophotometer (Agilent). The potential was con-
H NMR (300 MHz, CDCl ): d=0.90 (t, J=6.7 Hz, 6H; CH ), 1.43–
3 3
trolled with an Autolab PGSTAT101 potentiostat (Metrohm Auto-
lab). Spectra of the polymer films electrodeposited on an indium
tin oxide (ITO)–glass electrode were recorded in monomer-free sol-
utions. The same supporting electrolyte as the one used for poly-
merization was employed. ITO–glass electrodes were degreased by
means of sonication in acetone for 30 min and boiling in 0.1m
sodium hydroxide before polymer deposition. The background ab-
sorption was measured in a cell with a bare ITO electrode. For
both polymer films the UV/Vis spectra were determined in a poten-
tial range between À0.4 and 1.4 V with the 0.1 V steps. At each ap-
1.26 (m, 12H; CH ), 1.73–1.61 (m, 4H; CH ), 1.74 (s, 6H; CH ), 2.75–
2 2 3
3
3
2.63 (m, 4H; CH ), 7.02 (d, J=5.2 Hz, 2H; ArÀH), 7.15 (d, J=
2
3
3
8.4 Hz, 2H; ArÀH), 7.25 (d, J=5.2 Hz, 2H; ArÀH), 7.32 (dd, J=
4
4
8.4 Hz, J=2.1 Hz, 2H; ArÀH), 7.53 ppm (d, J=2.1 Hz, 2H; ArÀH);
13
C NMR (75 MHz, CDCl ): d=14.2, 22.8, 28.9, 29.4, 31.3, 31.8, 32.6,
3
34.3, 116.6, 123.4, 127.5, 128.7, 129.6, 129.8, 137.8, 138.6,
149.8 ppm; elemental analysis calcd (%) for C H OS : C 77.44, H
35
42
2
7.80; S 11.81; found: C 77.69, H 8.08, S 11.32.
plied potential, the quasi-equilibrium was allowed to be reached Acknowledgements
before the spectra were recorded.
1
13
The Polish National Centre of Progress of Explorations (grant no.
2012/05/B/ST5/00749) and the Wroclaw University of Technology
are gratefully acknowledged. K.O. thanks the German Academic
Exchange Service (DAAD) and the European Union as part of the
European Social Fund for a financial scholarship. The Wroclaw
Centre of Computing and Networking is acknowledged for gener-
ous allotments of computer time.
H and C NMR spectra were recorded in deuterated chloroform
CDCl with a Bruker DRX 300 Avance Instrument. Chemical shifts in
3
ppm (d) were referenced to the residual solvent signal. Elemental
analysis was performed with an Elementar Vario EL III. Topographi-
cal and morphological information was obtained with a JPK Nano-
wizard 3 atomic force microscope equipped with a Vortis control-
ler. Tapping mode was used for AFM imaging in air. Contact mode
was used for lithographical scratching of polymer films.
Keywords: conducting materials
·
density functional
Synthesis
calculations · electrochromic effect · polymers · xanthene
2
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3
3
3
4
ArÀH), 7.26 (d, J=4.3 Hz, 4H; ArÀH), 7.46 (dd, J=8.4 Hz, J=
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(
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Received: October 12, 2014
Revised: December 16, 2014
Published online on January 14, 2015
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim